Debugging it, i.e. removing a line a seeing what happens, gave the appearance that the margin rules of the desc were to blame. While fiddling around, I noticed that the font-size rule was irrelevant, so I removed it. Then when I reapplied the margin rules (written in another way), the layout was fixed. Further testing revealed that the font-size rule was to blame. Now all is well again. Or at least until someone finds the next bug.